1. Field of the Invention
The present invention relates to a display device, and more particularly, to a technique of enhancing strength and visibility of a miniaturized liquid crystal display device used in a mobile phone or the like.
2. Description of the Related Art
With respect to a liquid crystal display device, there has been a strong demand for the reduction of a thickness of a liquid crystal display panel along with a demand for the reduction of a profile size of a set while maintaining a screen at a fixed size. To decrease the thickness of the liquid crystal display panel, after manufacturing the liquid crystal display panel, an outer side of the liquid crystal display panel is polished to decrease the thickness of the liquid crystal display panel.
The liquid crystal display panel is constituted of a TFT substrate which has pixel electrodes, thin film transistors (TFTs), and the like formed thereon or a color filter substrate which has color filters formed thereon. A glass substrate which is served for forming the TFT substrate and the color filter substrate has a thickness thereof standardized to 0.5 mm or to 0.7 mm. It is difficult to acquire such standardized glass substrate from a market. Further, an extremely thin glass substrate gives rise to drawbacks on mechanical strength, deflection, or the like in manufacturing steps thereof thus lowering a manufacturing yield rate. As a result, after forming the liquid crystal display panel using the standardized glass substrate, an outer surface of the liquid crystal display panel is polished to decrease the thickness of the liquid crystal display panel.
The reduction of the thickness of the liquid crystal display panel gives rise to a drawback on mechanical strength. That is, there exists a risk that the liquid crystal display panel is broken when mechanical pressure is applied to a display screen of the liquid crystal display panel. To prevent such breaking of the liquid crystal display panel, as illustrated in FIG. 16, in assembling the liquid crystal display panel in a set such as a mobile phone, a front window (hereinafter referred to as face plate) is mounted on a screen side of the liquid crystal display panel.
The face plate is installed at a distance from the liquid crystal display panel in order to prevent an external force applied to the face plate from extending to the liquid crystal display panel. This, however, causes a different problem in a structure illustrated in FIG. 16 by degrading the quality of a displayed image as is described later. Meanwhile, structures that mechanically protect a liquid crystal display panel have been proposed. JP 11-174417A, for example, describes mechanically protecting the liquid crystal display panel with a sticky elastic member that is installed between the face plate and the liquid crystal display panel.
The related art illustrated in FIG. 16 gives rise to a drawback that an image appears as a duplicate image. FIG. 16 is a view describing the drawback by taking a reflective liquid crystal display panel as an example. In FIG. 16, an incident external light L passes a face plate, is reflected on the liquid crystal display panel, passes the face plate again, and enters human eyes. Here, the external light L is refracted on the face plate, but the refraction is ignored in FIG. 16.
A portion of light reflected on a screen P1 of the liquid crystal display panel is reflected on a lower surface Q1 of the face plate, is incident on a screen P2 of the liquid crystal display panel, and is reflected on the screen P2 of the liquid crystal display panel. When a viewer recognizes the light reflected on the screen P2 with his/her eyes, a phenomenon that an image appears as a duplicate image occurs. The description has been made by taking the reflective liquid crystal display panel as an example in FIG. 16, but the same goes for a transmissive liquid crystal display panel. That is, in the transmissive liquid crystal display panel, when light passes the liquid crystal display panel at an angle equal to an angle of reflection light on a screen P1 of the liquid crystal display panel, light is reflected on a lower surface Q1 of a face plate, and traces a path in the same manner as a path of the reflective liquid crystal display panel. The phenomenon which makes the image appear as a duplicate image deteriorates image quality.
The technique disclosed in JP 11-174417 A, on the other hand, is of attaching the face plate to a surface of the liquid crystal display panel with a sticky elastic member interposed between the two. A face plate that is discussed in the present invention is the type on which printing is performed in layers so that the face plate bears a specific frame pattern or a graphic pattern along its perimeter for the purpose of improving the design aesthetics of the display device. A frame printed onto a face plate affects in various ways the adhesion or the pressure-sensitive adhesion between the liquid crystal display panel and the face plate. The relation between the thickness of the pressure-sensitive adhesive material and the thickness of the printed frame or the like is a particularly important matter. However, JP 11-174417 A does not mention this and relevant matters.